The origins of the 15, 30, and 40 scores in tennis are believed to be medieval French. It is possible that a clock face was used on court, with a quarter move of the hand to indicate a score of 15, 30, and 45. When the hand moved to 60, the game was over.
The word "Tennis" came into use in English in the mid-13th century from Old French, via the Anglo-Norman term "Tenez", which can be translated as "hold!" [wikipedia]
